ll known that abnormal climatic
conditions influence behaviour; we see migrants retracing their flight
along the very course they travelled a short time previously--driven
headlong by the blizzard, that at least is what we say. But if the wind,
instead of being cold and from the north, is warm and from the west, do
they retrace their flight? I have not found it so. And if there be no
wind and the temperature is low, are they still affected? Again, I have
not found it so. When, as we commonly say, they fly before the storm,
some change takes place in their organic complex, some new impulse
receives stimulation or the former one lacks it. If, after Lapwings have
established themselves in their territories, the weather becomes
exceptionally severe, the birds collect together again in flocks and
revert to their winter routine; and under similar circumstances,
Buntings fail to sing and temporarily desert their territories. In such
cases it is clear that the impulse to seek isolation ceases for a time
to dominate the situation. The inference, therefore, is that atmospheric
changes bear some relation to the functioning of the instinct; but
whether it be temperature, or humidity, or the direction and velocity of
the wind, or a combination of two or more of these factors that supplies
the stimulus, we cannot tell.

The appropriate organic condition and the stimulus have then still to be
determined, and we must pursue our inquiry from the point at which the
impulse comes into functional activity. We will take a simple case, and
one free from complication.

Let us suppose that there is an area bereft of bird life, if it can be
so imagined, but in proximity to other inhabited areas. Into this area,
whilst in search of isolation, let us imagine that a Yellow Bunting
finds its way. After the manner of its race it establishes a territory
and occupies, let us say at a low computation, half an acre of ground.
It then obtains a mate, breeds, and rears offspring, two of which we
will assume are males. Reproduction ended, the birds desert the area,
and in the following spring, when the impulse again asserts itself,
parents and offspring seek again their former haunts. We now have three
males, each of which occupies half an acre, and each of which rears two
offspring--that is the position at the close of the second year. In the
third year the number will have increased to nine and the area occupied
to 4-1/2 acres; and so on in succeeding y